1. Field of the Invention
The present invention relates to a hydraulic actuator for actuating frictional engagement elements such as clutches or brakes of an automatic transmission and, more particularly, to a two-layered (double piston) hydraulic actuator structure of the type in which a first piston is fitted in a drum member whereas a second piston is fitted concentrically in the first piston and in which a single return spring biases both the first and second pistons.
2. DESCRIPTION OF THE PRIOR ART
In recent years, we have devised an automatic transmission which is equipped with the above-described type of hydraulic actuator structure as disclosed, for example, in Japanese Patent Laid-Open No. 141343. As shown in FIG. 6, this automatic transmission has its rear end portion rotatably supporting a drum member 2 which is fixed to an input shaft 1. In the drum member 2 is a first piston 3 hermetically and slidably mounted therein through O-rings 5 and 5', with its leading end engaging a first clutch C1. A second piston 6, with its leading and engaging a second clutch C2, is hermetically and slidably mounted concentrically within the first piston 3 through O-rings 7 and 7'. A return spring 10 is mounted between the back of the second piston 6 and a retainer 9 which is fixed to the input shaft sleeve portion 1a. The return spring 10 urges both the first piston 3 and the second piston 6 to the left in the drawing. The interior of the drum member 2 and the first piston 3 together form a hydraulic actuator 11 for the first clutch C1, and the interior of the first piston 3 and the second piston 6 together form a hydraulic actuator 12 for the second clutch C2. These two hydraulic actuators 11 and 12 are radially superposed to form a two-layered (double piston) hydraulic actuator unit 15.
The following table gives the positions of the two pistons for various transmission operating ranges in an automobile.
TABLE 1 ______________________________________ Range Cl C2 Stroke F A Pc ______________________________________ R x .smallcircle. s.sub.2 Fs(s.sub.2) Small High N x .smallcircle. D-1 .smallcircle. x s.sub.1 Fs(s.sub.1) Large Low D-4 .smallcircle. .smallcircle. s.sub.1 + s.sub.2 Fs(s.sub.1 + s.sub.2) Small Max. ______________________________________
Specifically, the first clutch C1 is engaged in the drive range D and released in the reverse range R and neutral range N, and the second clutch C2 is engaged in the R range and in the 4th speed of the D range and released in the remaining ranges. The first piston 3 applies a predetermined pushing force Fs(s.sub.1) to the first clutch C1, which is engaged at the 1st to 3rd speeds of the D range, against the force Fs of the return spring 10 corresponding a stroke s.sub.1. The first piston 3 has a large external diameter D.sub.1 and a relatively large pressure receiving area A.sub.1 so that the oil pressure Pc.sub.1 which acts upon the piston 3 is relatively low and is expressed by Pc.sub.1 =Fs(s.sub.1)/A.sub.1. The second piston 6 applies a predetermined pushing force Fs(s.sub.2), which corresponds to a stroke s, substantially equal to that of the first piston 1, to the second clutch C2 which is engaged in the R range. The second piston 6 has a small external diameter D.sub.2 which is substantially equal to the diameter of the internal portion of the first piston in which it slides, and has a relatively small pressure receiving area A.sub.2 so that the oil pressure Pc.sub.2 which acts upon the second piston 6 is relatively high and is expressed by the formula Pc.sub.2 =Fs(s.sub.2)/A.sub.2. At the 4th speed of the D range, moreover, the second clutch C2 is engaged while the first clutch C1 is also engaged. As a result, the second piston 6 moves with a stroke of (s.sub.1 +s.sub.2) so that the return spring 10 has a high spring load of (Fs(s.sub.1 +s.sub.2)). Since the piston 6 has the small pressure receiving area A, the oil pressure Pc.sub.2 which acts upon the piston 6 in range D-4 is at its maximum.
The return spring 10 is designed to provide a force by which the first piston 3 is quickly returned against the sliding resistance of the O-rings 5 and 5' when the range is shifted from D to R (i.e., D.fwdarw.&gt;R). In shifts of D.fwdarw.&gt;N ranges at a high speed, the spring force Fs is relatively high so that the first piston 3 moves smoothly against the sliding resistance of the O-ring 5 which is subjected to centrifugal force, to thereby prevent establishment of a trailing torque.
Thus, the oil pressure Pc.sub.1 to the hydraulic actuator 11 for the first clutch C1 can be held at a predetermined level to lighten the shift shocks in the shift of the N.fwdarw.&gt;D ranges, but the oil pressure Pc.sub.2 to the hydraulic actuator 12 for the second clutch C2 becomes too high for the accumulator to provide a sufficient cushioning function. As a result, high shift shocks may occur when the clutch C2 is engaged, i.e., in the shifting operations of N.fwdarw.&gt;R ranges and 3rd speed.fwdarw.&gt;4th speed.
The problems thus far described are encountered in an automatic transmission which has its second clutch C2 engaged in the reverse range and at the 4th speed and are also encountered in the automatic transmission which is disclosed in Japanese Patent Laid-Open No. 113165/1990, for example. In short, the problems are common to all double piston hydraulic actuator units having a shared return spring.